An Investigation into the Effects of Different Oxy-Nitrocarburizing Conditions on Hardness Profiles and Corrosion Behavior of 16MnCr5 Steels

This work is aimed to develop and optimize a gas ferritic oxynitrocarburizing treatment applied to an industrial 16MnCr5 component. The main effort has been to reduce the treatment time and the gas consumption by using a nitriding atmosphere that comprises nitrous oxide and ammonia. Special attention has been paid to improve the salt spray corrosion resistance with respect to the hardness requirements and the whole compound zone depth. Several changes have been applied to a base treatment by modifying the time and the temperature of each process phase, as well as the gas mixture and the flow of the nitrocarburizing atmosphere. Microhardness investigations and measurements of the compound layer's depth have been carried out on the treated specimens in order to evaluate the influences due to the variations of the process variables during the nitrocarburizing process. The introduction of nitrous oxide and ammonia allows reducing the nitriding time from 3 to 2 hours, and the purge time can be significantly reduced. Moreover, the increase of the nitrocarburizing temperature from 550 to 580°C allowed reducing the time of this phase from 4 to 3 hours. The post-oxidation time was moved from 3 to 4 hours in order to increase the thickness of the surface oxide layer and to improve the corrosion resistance

Lanthanides: a focused review of eutectic modification in hypoeutectic Al–Si alloys

A modified fibrous-like eutectic structure strongly improves the final mechanical properties of Al–Si foundry alloys, especially ductility. Beside widely used commercial eutectic modifiers such as Sr and Na, lanthanides appear to be a possible alternative in the eutectic Si modification process for hypoeutectic Al–Si casting alloys. All lanthanides have similar physical and chemical properties, such as density, melting point, and fading phenomena, which have been compared in the present review. They also show atomic radii close to the optimal atomic radius for a modifying agent. However, the microstructural evolution of eutectic Si is strictly related to the specific element and content of lanthanides, whose abundance, reserve, mining, production and market situations have been preliminary evaluated in this paper. The eutectic modification mechanisms induced by lanthanides are not well-discussed and clarified yet. The advantages and disadvantages of individual addition of lanthanides for chemical modification of hypoeutectic Al–Si alloys have been here critically reviewed. The use of lanthanides for eutectic Si modification in Al–Si alloys has been discussed from both effectiveness and economical point of views. Nowadays, the actual cost and their efficiency seem to make lanthanides still far to be used in foundry industry for commercial and large-scale applications

Optimizing the Heat Treatment Process of Cast Aluminium Alloys

The influence of heat treatment process on the microstructure and mechanical properties, and the distortion of a low-pressure die-cast AlSi7Mg0.3 alloy is reported. This work is aimed at optimizing the whole heat treatment process, including solution heat treatment, quenching and artificial ageing, of A356 alloy wheels, produced by low pressure die casting. In particular, the optimization process is focused on reducing time and temperature of solution and ageing heat treatments, and on selecting the right quenching medium, reducing the distortion of A356 alloy 17-in and 18-in wheels and obtaining the required mechanical properties. The time and temperature of solution treatment are normally chosen to dissolve coarse \u3b2-Mg2Si phase coming from the solidification process, especially where the solidification rate is slow as in the thickest zones of wheel, e.g. the hub and the spoke regions. Generally, the time required is about 45 minutes at the temperature of 540\ub0C, which is the typical solution temperature used for A356-type alloy. More time is however required at this temperature to change the morphology of eutectic silicon particles. This is aimed to improve the mechanical properties, especially ductility and fatigue properties, of wheels after further artificial ageing. The solutionising temperature is here selected to reduce the time of the heat treatment and to avoid any type of incipient melting of the material within an industrial tunnel furnace. The quenching is the most critical step in the sequence of heat-treating operations. The objective of quenching is to preserve the solid solution formed at the solution heat-treating temperature, by rapidly cooling to some lower temperature, usually near the room temperature. The most rapid quench rate, giving the best mechanical properties, can also cause unacceptable amounts of distortion or cracking in components. This is particularly true for aluminium wheels where the different thickness throughout the casting can produce distortion higher than 2-3 mm, critics for subsequent machining operations. The optimization process is here focused on the quenching rate, which is varied by changing the temperature of the quenchant, in order to reduce the wheels\u2019 distortion and to guarantee the appropriated supersaturation level of atoms for subsequent ageing treatment. Therefore, the temperature of forced water, used as quenchant, has been varied in the range of 50 to 95\ub0C, and the distortion level and the hardness of the wheels systematically measured. A temperature of 95\ub0C is observed to be the optimum solution, but useless from an industrial point of view. The extreme vapour produced by the boiling water can compromise the automatism for handling of wheels and delay too much the cooling of the wheels after solutioning. Therefore, the best compromise between distortion and mechanical properties and productivity has been revealed to be at a temperature of 75\ub0C. The time and temperature of ageing are considered in the present work in order to reach an underageing temper of the material, which is typical for the manufacture of wheels to improve impact and fatigue properties. The influence of the different painting processes, generally carried out at a temperature of 180\uf7190\ub0C for several minutes after ageing and subsequent machining operations, have to be considered to determine the final mechanical properties. These temperatures are typical temperature for ageing treatment of cast aluminium alloys, such as the A356-type alloys. Therefore, the present study takes into account the painting temperatures and times to optimize the ageing treatment of the wheels, reaching the desired mechanical properties. This study develop an optimization approach of the whole heat treatment process of cast aluminium alloy wheels. The work evidences the typical problems and targets of wheels\u2019 producers, suggesting an integrated approach to improve the productivity and the quality of castings. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring during heat treatment; while hardness and tensile testing measurements have been carried out to monitor the evolution of the mechanical properties after each step of T6 heat treatment

Constitutive and stochastic models to predict the effect of casting defects on the mechanical properties of High Pressure Die Cast AlSi9Cu3(Fe) alloys

The effect of casting defects on mechanicalproperties of a high-pressure die-castAlSi9Cu3(Fe) alloy is reported. A series of Ushapedstructural components are cast usinga combination of injection parameters andpouring temperatures in order to generatedifferent types and amount of casting defectsthroughout the casting. It has been found thatcastings contain defects, primarily pores andoxides, and that the presence and distributionof these defects are highly sensitive to theprocess conditions. Moreover, significantvariations of the defect distribution have,however, been found in castings producedunder the same conditions, indicating thestochastic nature of defects in die castings.The tensile properties are affected by theamount and distribution of defects and aredetermined by the defect area fraction. Theinfluence of casting defects on mechanicalproperties are investigated through atheoretical verification based on constitutiveand stochastic models. The analyticalapproach, based on the Ghosh constitutivemodel of tension instability, correctly indicatesthe trends of the experimental results, whilethe Weibull statistics evidences how the scaleparameter and the Weibull modulus arestrongly affected by the casting conditions.An integrated stochastic-analytical approachis then proposed and it appears to beapplicable to describe the tensile properties interms of fractographic defects and cumulativefailure probability Pi

Evaluation HPDC Lubricant Spraying for Improved Cooling and Die Protection

This study tries to find out a better cooling and temperature homogenization as well as better die protection on high-pressure die-casting (HPDC) spray lubrication. Test procedures have been set up to study the Leidenfrost point (LFP), contact angle (CA), film thickness and protection from die soldering of lubricants typically applied into the die surfaces during HPDC process. Five different lubricants have been studied as well as the influence in different controllable process parameters (type of die material, oxidation of the surface, temperature, roughness, droplet diameter, water hardness and lubricant concentration). The increase of the LFP, avoiding film boiling regime, and a reduced CA, improve the cooling and film ability of die surface during spraying. The best chemistry exhibits high LFP, shows an increased thickness of the formed film and is more effective preventing the sticking of the aluminum part to the die surface. Thermogravimetric analysis shows better thermal properties for lubricants with anti-sticking performance. The study performed and the test protocols set up result in a better insight of the involved phenomena and allow selecting the most favorable operating window for HPDC lubrication

Increasing the Liquidus Temperature by Employing the Controlled Diffusion Solidification (CDS) Process: A Potential Route to Improved Castings

Abstract Recent theories suggest the existence of an incubation time, over which a liquid alloy prepares for nucleation by decomposing into compositional fluctuations. Accordingly, in a recent work by the present authors, the solidification path of a Controlled Diffusion Solidification (CDS) mixture was calculated. The calculated CDS path begins at a higher liquidus temperature comparing to conventional solidification and the fraction solid values are achieved at a relatively higher temperature. To provide information on the CDS mechanism and physical structure of the CDS mixture in the mushy zone, Al-7.8Zn-2.6Mg-2Cu alloy was solidified, in this study, via conventional and CDS process in the presence and absence of recalescence. Typical grain structures obtained via the two solidification conditions is characterized using Electron Back Scattered Diffraction. Results showed that the nucleation continues to occur in the presence of recalescence, while it is suppressed in its absence. According to the two step nucleation theory, the increase in the nucleation temperature causes sufficient recalescence in the mixture, allowing the unnucleated liquid phase to decompose into chemical fluctuations and prepares for further nucleation. As a result, in the presence of recalescence, nucleation in a CDS mixture is not as readily halted as during the conventional solidification, which is in contradiction with the recent theories developed based on the classical theory of nucleation

High-Temperature Behavior of High-Pressure Diecast Alloys Based on the Al-Si-Cu System: The Role Played by Chemical Composition

Al-Si-Cu foundry alloys are widely applied in the form of high-pressure diecast components. They feature hypo- or nearly eutectic compositions, such as AlSi9Cu3(Fe), AlSi11Cu2(Fe), and AlSi12Cu1(Fe) alloys, which are used in the present study. Diecast specimens, with a thickness of 3 mm, were used for tension tests. The short-term mechanical behavior was characterized at temperatures from 25 up to 450 C. At temperatures above 200 C, the tensile strength properties (YS and UTS) of the investigated alloys were severely affected by temperature, and less by chemical differences. Material hardness and ductility indexes better highlighted the differences in the mechanical behavior of these age-hardenable alloys and allowed us to relate them to the microstructure and its changes that took place at test temperatures. Thermodynamic calculations were found to be useful tools to predict phases formed during solidification, as well as those related to precipitation strengthening. By means of the performed comprehensive material characterization, deeper knowledge of the microstructural changes of Al-Si-Cu foundry alloys during short-term mechanical behavior was obtained. The gained knowledge can be used as input data for constitutive modeling of the investigated alloys

Correlation between processing and quality of aluminium alloy castings (Correlazione tra parametri di processo di fonderia e qualità dei prodotti)

The influence of processing and process parameters plays a key role for the Aluminium foundry and transport industries as it affects the quality and soundness of the cast products. Particularly, the choice of a process chain in Aluminium foundry, otherwise of process parameters, influences the reject rates, hence casting costs, the process yield and the production rate. The process chain in Aluminium foundry is a complex sequence of processes and the final casting quality depends on many parameters. Several aspects of this subject are still not fully understood. The motivation of the research presented in this doctoral thesis work was, therefore, to fill this gap in knowledge. The study has aimed at understanding the influence of various process and process parameters of foundry on the quality of aluminium alloy castings and, in particular, Al-Si based castings. A literature review and a sufficient background of previously reported results on the influence of processing and process parameters on the quality of aluminium alloy castings, physical fundamentals as well as industrial challenges, motivation and goals were carried out. Special attention in Aluminium process chain has been given to: The modification of aluminium-silicon cast alloys: before casting aluminium alloys, the molten metal can be treated in order to improve the microstructure and properties of alloys by addition of small quantities of certain "modifying" elements. The pouring of molten metal into the mould: this is one of the critical steps in foundry technology, since the behaviour of the liquid and its subsequent solidification and cooling determine whether the cast shape will be properly formed, internally sound and free from defects. The chill casting processes, such as gravity, low-pressure and high-pressure die casting processes: the essential feature of chill casting is the use of permanent metal moulds, into which the molten alloy is either poured directly or injected under pressure, giving rise to the separate processes of gravity and low/high pressure die casting. Permanent moulds offer obvious advantages in terms of simplicity of production for large quantities of parts, but are subject to limitations yet to be discussed. The heat treating process applied to high-pressure die castings: conventional die castings are utilised to produce many products but unfortunately the presence of porosity limits the application. In addition to porosity, the microstructure inherent with conventional die casting could not meet the mechanical requirements needed for many applications. Subsequent heat treating, which can positively alter the microstructure, is rarely possible due to defects that emerge during thermal processing, such as blistering

High-Pressure Die-Cast AlSi9Cu3(Fe) Alloys: Models for Casting Defects and Mechanical Properties

The effect of casting defects on the tensile properties of high-pressure die-cast AlSi9Cu3(Fe) alloys is reported. A series of U-shaped structural components has been die-cast using a combination of injection parameters and pouring temperatures in order to produce different types and amount of casting defects throughout the casting. The results reveal how the die-castings contain defects, primarily pores and oxides, and their presence and distribution are highly sensitive to the process conditions. The tensile properties are affected by the amount and distribution of defects and can be determined by the defect area fraction. The influence of casting defects on the tensile properties are investigated through a theoretical verification based both on constitutive and stochastic models. The analytical approach, based on the Ghosh constitutive model of tension instability, correctly indicates the trends of the experimental results, while the Weibull statistics evidences how the scale parameter and the Weibull modulus are strongly affected by the casting conditions

La presenza del Fe in leghe Al-Si da fonderia Parte 3 \u2013 Metodi alternativi di neutralizzazione

In questo lavoro vengono presentati alcuni metodi alternativi all\u2019alligazione per neutralizzare gli effetti deleteri prodotti dalla precipitazione di \u201caghetti\u201d ricchi in Fe in leghe Al-Si da fonderia. Una tecnica proposta riguarda le fasi preliminari alla colata, durante cio\ue8 il trattamento del metallo liquido, e si basa sul surriscaldamento del bagno liquido poco prima delle operazioni di colata. Tecniche alternative possono essere i trattamenti termici di non-equilibrio, cio\ue8 trattamenti post colata condotti a una temperatura di poco superiore alla Tsolidus. Questi metodi, seppur attualmente poco diffusi, hanno una potenzialit\ue0 elevata nella dissoluzione e frammentazione della fase lamellare \u3b2-Al5FeSi, con conseguente miglioramento delle caratteristiche finali del getto